1. Field of the Invention
This invention relates to an apparatus for incorporating a lock plate in the cassette case body of a video cassette which accommodates a magnetic tape. This invention also relates to a magnetic tape cassette which is composed of a cassette case body and a guard panel and which is provided with a lock plate for locking the guard panel in its closed position.
2. Description of the Prior Art
Magnetic tape cassettes comprising a case which accommodates a magnetic tape have heretofore been used widely. FIG. 4 is a perspective view showing a video cassette, which is one type of magnetic tape cassette. With reference to FIG. 4, the video cassette comprises a cassette case body 3 having a case 1, which is composed of an upper case half 1A and a lower case half 1B and which accommodates a magnetic tape 2. The cassette case body 3 has a surface 1a along which the magnetic tape 2 moves, and a guard panel 6 which is provided on the surface 1a and which comprises a guard member 4 positioned parallel to the surface 1a. The guard panel 6 has a shaft 7 at its side and is rotatably mounted on the cassette case body 3. The shaft 7 is engaged with a hole 8 of the case 1 so that the guard member 4 can be moved between a closed position in which the portion of the magnetic tape 2 located on the surface 1a is covered and an open position retracted from the surface 1a in which the portion of the magnetic tape 2 located on the surface 1a is laid bare.
Also, a lock plate, which locks the guard panel 6 in its closed position when the video cassette is not being used, is located in the cassette case body 3. FIG. 5 is an exploded perspective view showing the major parts of the video cassette. Specifically, as shown in FIG. 5, a lock plate 10 comprises a sheet-like plate body 11, a first protrusion 12 and a second protrusion 13 which protrude from the plate body 11 in a direction approximately normal thereto, a shaft 14 which is combined integrally with the plate body 11 and extends horizontally, and a torsion spring 15 having a coiled portion 15a engaged with the shaft 14. The lock plate 10 is fitted in a lock plate mount 20, which is located at a side edge of the lower case half 1B. A side wall 24 of the lock plate mount 20 has holes 21 and 22. The lock plate 10 is fitted in the lock plate mount 20 with the protrusions 12 and 13 respectively engaged with the holes 21 and 22. At this time, one leg portion of the torsion spring 15 is located so that it is in contact with a rear surface of the plate body 11 (i.e. the surface opposite to the surface on which the protrusions 12 and 13 are provided). The other leg portion of the torsion spring 15 is located so that it is in contact with an inner wall surface 23 of the lock plate mount 20, which inner wall surface faces the rear surface of the plate body 11. Accordingly, the torsion spring 15 forces the protrusions 12 and 13 to project from the lock plate mount 20 through the holes 21 and 22. A recess 5, which is capable of engaging with the protrusion 13, is formed in the inner side surface of the guard panel 6. The protrusion 13, which is forced to protrude outwardly from the lock plate mount 20, engages with the recess 5 when the guard panel 6 is to be locked in the closed position. Therefore, the guard panel 6 is prevented from rotating to the open position. The protrusion 12 is not covered by the guard panel 6. When the video cassette is fitted into a cassette deck, a member of the cassette deck pushes the protrusion 12, which causes the lock plate 10 to be pushed into the cassette case body 3 and thereby releases the lock of the guard panel 6.
In order to incorporate the lock plate 10 in the cassette case body 3, a plate holding means 30 as shown in FIGS. 6A and 6B and a fitting means 40 as shown in FIG. 7 are provided.
Specifically, a plurality of lock plates 10, 10, . . . are fed one after another by a parts feeder (not shown) to the plate holding means 30. As shown in FIG. 6A, a lock plate 10 is held upright on the plate holding means 30 with the plate body 11 inserted in a holding part 33 of the plate holding means 30. The plate holding means 30 has grooves 31 and 32 for receiving the protrusions 12 and 13. Thereafter, a torsion spring 15 is fed toward the lock plate 10 by a parts feeder (not shown) and engaged with the shaft 14 of the lock plate 10. The plate holding means 30, on which the lock plate 10 is being held, is then conveyed to a position at which a lower case half 1B of the cassette case is located. At said position, the lock plate 10 is grasped by the fitting means 40 composed of a pair of grasping members as shown in FIG. 7. The lock plate 10 is thus taken out of the plate holding means 30, and fitted in the lower case half 1B of the cassette case body.
However, the plate holding means 30 holds the lock plate 10 upright. Therefore, when the lock plate 10 is taken up from the plate holding means 30 by the fitting means 40, moved horizontally and then moved down to the lower case half 1B, the protrusions 12 and 13 of the lock plate 10 strike against the surface of the wall of the lock plate mount 20. Therefore, the lock plate 10 cannot be fitted smoothly into the lower case half 1B. In order to prevent this problem, before the fitting means 40 fits the lock plate 10 into the lower case half 1B, it must rotate the lock plate 10 from the position indicated by the chained line in FIG. 7 to the position indicated by the solid line. For this purpose, the fitting means 40 must be provided with a rotation mechanism for changing the inclination of the lock plate 10. As a result, the fitting means 40 has a complicated mechanism and is large in size. Also, the unit which supports the fitting means 40 must be very rigid and will be large. Such an apparatus for incorporating a lock plate in a video cassette is not suitable for quick incorporation of lock plates.
The problems of conventional lock plates will be described hereinbelow. FIG. 12 is a perspective view showing a conventional lock plate. FIG. 13 is an enlarged perspective view showing the projections on the conventional lock plate. FIG. 14 is a sectional view showing the conventional lock plate.
With reference to FIG. 12, projections 16 and 17 are formed on a rear surface 11a of the plate body 11, which surface comes into contact with a leg portion 15c of the torsion spring 15. The projections 16 and 17 are formed side by side in the direction along which the shaft 14 extends. The leg portion 15c is fitted in a groove 18 defined by the projections 16 and 17.
Before the lock plate 10 is incorporated in the aforesaid cassette case body 3, the torsion spring 15 is engaged with the plate body 11. However, with the conventional technique, failures readily arise during the engagement of the torsion spring 15. Specifically, as shown in FIG. 12, the torsion spring 15 is moved along the shaft 14 in the direction indicated by the arrow A until a coiled portion 15a engages with the shaft 14. At this time, as shown in FIG. 14, the torsion spring 15 is rotated by an angle .theta.1 from a predetermined mounting position so that the leg portion 15c does not strike against the projection 17 located on the upstream side as viewed in the direction along which the torsion spring 15 moves. However, as shown in FIG. 13, the projection 17 has a rectangular parallelopiped shape. Therefore, if the position to which the torsion spring 15 is rotated during fitting to the shaft 14 fluctuates or if the angle between the leg portions 15b and 15c fluctuates in the process of forming the torsion spring 15, the leg portion 15c will readily strike against the upper edge of the projection 17, and it cannot therefore be fitted accurately in the groove 18. Also, if the relationship between the positions of the torsion spring 15 and the projections 16, 17 fluctuates during engagement, the leg portion 15c will get stuck on the projections 16 and 17, and therefore the torsion spring 15 cannot accurately be engaged with the shaft 14.